The 5' end of the mRNA encoding one of the late major structural protein fibers has been precisely localized on the Ad2 genome and its nucleotide sequence has been determined. Fiber mRNA species are heterogeneous, basically differing in their leader sequences at the 5' end. Studies will be conducted to answer the questions as to how and where mRNA processing takes place and whether the intervening sequences are excised as large segments of RNA or removed sequentially in small pieces. We have already cloned and analyzed the cDNA from some selected species of fiber mRNA. Other species of these mRNA populations will be cloned. Various restriction enzyme fragments obtained from these clones and representing the leader sequence regions will be used as primers for reverse transcription. Additionally, the cloned DNA pieces will serve as hybridization probes to select RNA processing intermediates both from the nucleus and the cytoplasm of infected human cells. The genomic sequences around the various splice points would be derived. This study will thus provide the primary sequences at the splice points. Comparison of the mRNA sequences with the genomic sequences may allow an assessment of the structural features of the primary sequence responsible for the mechanism of splicing. Comparison of the nuclear RNA species with that of cytoplasmic RNAs may give some understanding of the cellular site and mechanism of mRNA processing, and may reveal some significance of splicing in transcriptional control and mRNA processing.